In a SS or CDMA communication system, a signal to be transmitted covers a very widely spread frequency band as opposed to the ranges used in conventional communication systems and, therefore, less subjected to influence by noise. This and other advantages of the SS or CDMA communication systems have made them very attractive for consumers and made consumers reconsider their positions, especially as complicated arrangements and high costs of SS or CDMA communication systems became things of the past with the advent of technology improvements.
In a SS or CDMA communication system, a series of codes with a predetermined high bit rate, for example a pseudo random noise code series, is used for what is called the spread spectrum modulation of a carrier wave with narrow frequency band obtained by modulating information to be transmitted by a base band signal. The spread spectrum modulation can be exercised by different methods, a direct sequence method including. The direct sequence method provides for modulating a binary code with noise-like qualities on to a carrier. The spread signal is thereby obtained. The most desirable selection of a secure code requires, a long sequence, and a long sequence will require long acquisition time at the receiving side.
In the system of the kind discussed, a receiver is provided with a demodulator to demodulate the incoming spread signals. This operation is dependent on having at the receiving side a replica (or a match, or a pattern) of the pseudo random noise code sequence which is being transmitted. The replica should be in phase with the incoming sequence. Every time the coded pattern of the spread signal corresponds to the pattern, or more specifically, when the pattern is matched with the pseudo random noise code pattern at the time the digital signal is modulated to the spread signal, the spread signal is picked up as the information signal and is transferred to a further processing. Only the interrelating components of the transmitted signals are obtained in this case, since the received signal is multiplied by the pseudo random noise code in perfect analogy with that in the modulation inasmuch as there is no interrelation between different pseudo random noise codes. The above mentioned acquisition time is lengthy since the incoming sequence must be phase and code searched until correlation is attained. Phase searching extends acquisition time.
The process of phase searching can be expedited using tapped delay lines or matched filters. This filter is one of the most important component of a SS and CDMA communication system. In a conventional matched filter, in order to match the increase in length of a random sequence, the number of additions must also be increased. With the increase in the number of additions, the speed of passing the random sequence therethrough becomes limited and should now be increased since it affects the processing time of the system. In order to maintain an acceptable processing time, the speed of passing through the additions must be increased which in turn increases the energy consumption of the system and decreases its efficiency.
In a SS System or a CDMA system, there are different ways of performing code acquisition. These include the use of a correlator and a matched filter. A correlator proper that can be referred to as an active correlator has a simple structure and provides for multiplication of the received pseudo noise signal with noise by the pseudo noise reference at the receiving side and further integration, the result of which integration is used for making an acquisition decision by comparison with a threshold. An active correlator has a basic limitation on the search speed due to a specificity of the multiply-and-integrate type of correlation.
However, the search rate of a direct sequence code acquisition technique can be significantly increased by replacing the multiply-and-integrate operation with a passive correlator device such as a matched filter. A matched filter has a similar architecture as that of a finite impulse response (FIR) tapped-delay-line, or transversal filter. A matched filtering can be implemented either as a continuous time or discrete time operation using such state-of-the-art technologies as charge coupled devices, surface acoustic wave structures, and MOS structures. Generically, a matched filter is a passive device that maximizes the signal-to-noise ratio at its output when the signal at its input is embedded in additive white Gaussian noise. The use of matched filters is described in the art, for instance, in M. K. Simon, J. K. Omura, R. A. Scholtz, and B. K. Levitt "Spread Spectrum Communications" Vol. III, 1 printed by Library of Congress Cataloging in Publication Data 1985.
Conceptually, the implementation of a matched filter acquisition system for a finite length pseudo noise waveform can be visualized in the form of a tapped delay line followed by a passive filter matched to a single pseudo noise chip waveform. The architecture of a conventional matched filter 10 shown in FIG. 1 includes a delay line 12.sub.1, 12.sub.2, . . . , 12.sub.n, 12.sub.n+1 with taps to a multiplication stage 14.sub.1, 14.sub.2, . . . , 14.sub.n-1, 14.sub.n, and an adder 16. It can be appreciated that a square block T is the delay of T; X.sub.1, X.sub.2, . . . , X.sub.n, X.sub.n+1 is the input sequence, and a.sub.1 . . . a.sub.n-1, a.sub.n is the random code. A digital implementation of a matched filter uses a shift register, a holding register loaded with the random code, and a comparison and summation block. In this case, the contents of the shift register which holds the signal samples digitized to k bits, k an integer, and of the holding register which permanently contains the fixed segment of the code used for comparison are multiplied by each other stage by stage, generating "x.sub.i ", i=1.about.n, if the stages match and "-x.sub.i ", i=1.about.n, if they don't, and summing the resulting set of "x.sub.i " or "-x.sub.i ", i=1.about.n. The larger is the value of n the random code sequence, the longer the addition is performed. In the technology of SS or CDNA communication, the length n of a matched filter may exceed 1000 elements of delay in the delay line. A considerable number of multiplications and additions affect power consumption and hardware complexity. When a substantial number of additions is necessary for the adder 16, the speed is limited by the material used and the number of delays (frequently referred to as the number of taps). For instance, in a commercially available matched filter wherein a CMOS is used, the total number of taps for the matched filter cannot exceed 64. The input sampling speed is 30 Mchips/s. (See G. J. R. Povey and P. M. Grant, "Simplified Matched Filter Receiver Design for Spread Spectrum Communication Application", Electronics & Communication Engineering Journal, April 1993.)
It is therefore an object of the present invention to provide a matched filter for a spread spectrum or CDMA system that does not have the shortcomings of conventional matched filters.
It is another object of the present invention to provide a matched filter for a SS or CDMA system that utilizes a differential value from the matched filter as its matched output of the filter.
It is a further object of the present invention to provide a differential matched filter for a SS or CDMA system that can save approximately half of the number of additions necessary.